INTRODUCTION

Cerebrospinal fluid (CSF) is formed within the ventricles and circulates in the subarachnoid space (between arachnoid and pia matter) and in the ventricles.

The total volume of CSF in adults ranges from 90 to 150 mL and in neonates from 10–60 mL. It is a medium for transfer of substances from brain and spinal cord into the blood.

Functions of CSF is listed in Box 52.1

IMPORTANCE OF CSF EXAMINATION

Analysis of the CSF is of diagnostic importance in conditions like meningitis or primary/metastatic tumor of CNS with CSF involvement.

COLLECTION OF CSF

CSF is usually obtained by lumbar puncture (LP) using an LP needle under strict aseptic conditions.

Lumbar Puncture Needle (Fig. 52.1)

The needle measures 10–12 cm in length and preferably a 22 gauge needle is used. In children, a shorter needle is used. It has a needle and a stilette. The stilette has a pin which fits into the slot of the head of the needle and helps to keep the needle patent.

Fig. 52.1: Lumbar puncture needles with stilette (disposable needle with green plastic hub above)


Sites

Lumbar puncture:

In adults, CSF is normally collected in the midline of the lower back in the 3rd lumbar space and in children in the 4th lumbar space.

Cisternal puncture:

It is done in spinal cord block, vertebral deformity or infections in the tissues where lumbar puncture is usually done.

Ventricular puncture:

Directly from ventricles by operation or burr hole. It is performed in infants who have open fontanelle by transfontanelle puncture holes.

Shunt drainage


CSF Pressure

The normal CSF pressure in adults is 90–180 mm of water in the lateral position, while in infants and children it ranges from 10–100 mm of water reaching the adult level by 6–8 years.

•• Causes of raised CSF pressure: Meningitis, cerebral edema, mass lesions in brain, etc.

•• Causes of decreased CSF pressure: Dehydration, circulatory collapse, etc.

Method of Collection

Normally CSF is submitted to the laboratory in three or occasionally four sterile test tubes. The amount of CSF collected should not exceed 6 to 8 mL.

•• Tube 1: For estimation of protein and glucose and serology.

•• Tube 2: Used for preparation of smears to stain with the Gram stain or other stains and for culture and sensitivity.

•• Tube 3: For cell counts and differential counts.

•• Tube 4 (if indicated): For special tests such as the cryptococcal antigen, serologic test for syphilis, molecular tests or other serologic studies, and cytology.

Note:

•• CSF should be examined immediately within 1 hour because delay causes enzymatic destruction of cells. Refrigeration may inhibit bacterial growth but does not protect the cells from undergoing lysis.

•• CSF sample meant for culture, should not be refrigerated, because the most common causative agent for meningitis(Neisseria) is destroyed when exposed to cold temperature.

Indications for Lumbar Puncture: Refer to Box 52.2.

Contraindications for Lumbar Puncture: Refer to Box 52.3.

Complications of Lumbar Puncture


•• Herniation of cerebellum through the foramen magnum due to raised intracranial pressure.


•• Hematoma, either extradural or subdural.


•• Introduction of infection by the LP needle through the infected skin or subcutaneous tissue.


•• Post-puncture headache.


•• Failure to obtain CSF (dry tap).

EXAMINATION OF CSF

Physical Examination 

CSF pressure: 

The normal CSF pressure in adults is 90–180 mm of water in the lateral position, while in infants and children it ranges from 40–100 mm of water reaching the adult level by 6 to 8 years.

•• Causes of raised CSF pressure:

  1. Meningitis,
    1. cerebral edema, 
    2. and mass lesions in brain.

•• Causes of decreased CSF pressure:

  1. Dehydration and 
    1. circulatory collapse.

Color and appearance:

Normal CSF is clear and watery.

•• Turbidity of CSF is due to pus or RBCs.

•• Xanthochromia (Box 52.4):

Differentiation of subarachnoid hemorrhage from a traumatic puncture

–– The presence of blood in CSF may be due to trauma during lumbar puncture or subarachnoid hemorrhage.

–– In traumatic tap, first few drops are hemorrhagic and subsequent ones are clear, while in subarachnoid hemorrhage blood is uniformly mixed with CSF.

–– After centrifugation, the supernatant fluid is clear with a traumatic tap, whereas the supernatant appears xanthochromic (a faint pink, orange or yellow color caused by release of hemoglobin from hemolyzed red blood cells) in subarachnoid hemorrhage.

•• Clot formation:

Normal CSF does not clot. When blood-brain barrier is disturbed, fibrinogen appears in CSF. Fibrinogen gets converted to fibrin and forms clot.

•• Causes of fibrin clot:

–– Meningitis

◆◆ In tuberculous meningitis, the clot is fine, delicate and typically described as cob-web appearance.

◆◆ In purulent meningitis, large clot is formed.

–– Tumors of CNS

–– Polyneuritis.

Microscopic Examination

Total cell count:

Normal CSF usually contains no cells, although cell count of 0–5 lymphocytes/μL is considered as normal. An increased cell count in CSF is known as pleocytosis. Total cell count should be done immediately on undiluted CSF, since after a few hours, pus cells stick to each other and to the sides of the tube or degenerate.

Cell counts are performed in a manual counting chamber, either in a Fuchs–Rosenthal or improved Neubauer chamber. CSF findings in various meningitis is presented in Table 52.1.

Significance

•• Neutrophils are increased in acute pyogenic meningitis.

•• Lymphocytes are increased in viral, syphilitic, tuberculous and fungal meningitis. Increased cell count should always be confirmed by bacterial or serological tests. India-ink preparation is used for diagnosis of cryptococcal meningitis. In tuberculous meningitis, acid fast stain can detect tuberculous bacilli.

Wet Preparation

Different methods for wet preparation include:

•• Cytocentrifuged preparation (1000 RPM for 5 minutes) provides excellent cell recovery and is the method of choice.

•• Direct smears are prepared from the sediment after adding a drop of toluidine blue stain.

•• Filtration method may be followed.

Abnormal Cells in CSF

•• Relative pleocytosis: Atypical forms may be confused with leukemic cells.

•• Acute leukemia: CNS involvement by acute (especially lymphoblastic) leukemia may show blast cells in CSF.

•• Chronic leukemia: May show mature or immature cells.

•• Lymphoma: Cell type depending on the type of lymphoma.

•• Malignant cells: Carcinoma cells appear large with atypical nuclear features.

•• Primitive neuroectodermal cells: These cells may be found in clusters with formation of rosettes.

Biochemical Evaluation

•• Proteins are elevated in meningitis and glucose level is reduced due to utilization by the microbes. These changes are more marked in pyogenic meningitis.

•• Chloride reduction in tuberculous meningitis is due to general chloride deficiency because of dehydration rather than any specific effect of mycobacteria.

CSF analysis is of diagnostic importance in conditions like meningitis or primary/metastatic tumor of CNS with CSF involvement.

•• CSF is obtained by lumbar puncture using an LP needle under strict aseptic conditions.

•• Lumbar puncture has both diagnostic and therapeutic indications.

•• Examination of CSF consists of physical, microscopic and biochemical analysis.

•• Microscopically, neutrophils are increased in acute pyogenic meningitis and lymphocytes are increased in viral and tuberculous meningitis.